1. Field of the Invention
The present invention relates to a surface emitting laser apparatus usable as a light source in the fields of large-capacity optical communication, optical interconnection, optical information processing, parallel optical recording, and the like, its fabrication method, and its driving method.
2. Description of the Related Background Art
The arrangement of arrayed laser devices for parallel transmission of optical information has been recently studied to achieve large-capacity optical communication and optical interconnection. Also, a structure utilizing plural recording light sources has been studied to further advance the printing speed of laser beam printers. Particular interest has been shown in the use of vertical cavity surface emitting lasers (VCSELs) as light emitting devices suitable for the array arrangement of those light sources.
Recently, the following surface emitting laser has been energetically researched and developed to improve the performance of the laser. In such a surface emitting laser, a semiconductor layer containing aluminum (Al) is provided near the active layer, and this Al-containing layer is selectively oxidized to form a current confinement structure. An example of such a laser is disclosed in xe2x80x9cAppl. Phys. Lett. 1995, 66, (25), pp.3413-3415xe2x80x9d. FIG. 1 is a schematic diagram of this structure, which includes an n-type semiconductor multi-layer mirror 1001, an active layer 1003, a semiconductor layer 1005 containing Al, a p-type semiconductor multi-layer mirror 1007, and a p-side electrode with an opening.
In the above structure of FIG. 1, the Al-containing semiconductor layer 1005 is selectively oxidized, and its peripheral portion is altered so as to form an insulating layer 1006 that is chiefly formed of AlxOy to form the current confinement structure. Due to the current confinement structure, current can be effectively and efficiently injected into the active layer 1003, and hence, low threshold current and single-mode oscillation can be achieved in this type of surface emitting laser.
The resistance of that laser is, however, high since the resistance of the p-type semiconductor multi-layer mirror 1007 is large and the current passing region in the current confinement structure is small in area (about several micrometersxc3x97several micrometers, or twenty (20) square micrometers). As a result, the laser inevitably has disadvantages that its operation voltage needed to obtain a desired light power increases and that its characteristics are lowered due to a great amount of heat generated in the device.
Several methods have been researched and developed to reduce the resistance of the device. One of those methods is a method of injecting a current through a path escaping the semiconductor multi-layer mirror, which is disclosed in xe2x80x9cElectron. Lett., 1995, 31, (11), pp.886-888xe2x80x9d. FIG. 2 is a schematic diagram of this structure, which includes an n-type substrate 2001, ann-type semiconductor multi-layer mirror 2003, an active layer 2005, a semiconductor layer 2007 containing Al, a p-type contact layer 2009, and an undoped semiconductor multi-layer mirror 2011. In this example, the Al-containing semiconductor layer 2007 is selectively oxidized, and its peripheral portion is altered so as to form an insulating layer 2008 chiefly formed of AlxOy to form the current confinement structure. Further, a p-side electrode 2013 is deposited on the p-type contact layer 2009, and an n-side electrode 2015 is deposited on the bottom surface of the n-type substrate 2013. The p-side electrode 2013 is formed on the contact layer 2009 in this structure. The p-type semiconductor multi-layer mirror 2011 having a high resistance is not present in a current path between the n-side electrode 2015 and the p-side electrode 2013.
In another method for reducing the resistance of the device, a p-type semiconductor multi-layer mirror, a semiconductor layer containing Al, an active layer, and an n-type semiconductor multi-layer mirror are layered on a p-type substrate, etching is conducted from the side of the n-type semiconductor multi-layer mirror to expose the side of the Al-containing semiconductor layer and form a protruded pole, and the Al-containing semiconductor layer is oxidized toward its central portion to construct the current confinement structure. In this structure, the resistance can be reduced since the area of a current flow through the p-type semiconductor multi-layer mirror can be enlarged.
In the above-mentioned conventional surface emitting lasers, however, an increase in the resistance due to a small area of the current flow through the current confinement structure is not considered, though the resistance can be decreased by escaping the p-type semiconductor multi-layer mirror as a current path.
It is an object of the present invention to provide a surface emitting laser apparatus in which its operation voltage and consumption electric power can be reduced due to its low resistance, a common-anode construction is possible, a high-speed modulation can be achieved, and a construction capable of wavelength division multiplexing can be readily attained, its fabrication method, and its driving method.
The present invention is generally directed to a surface emitting laser apparatus having at least a light-emitting device, which includes a substrate, a cavity structure of the light-emitting device including a first n-type semiconductor multi-layer mirror, a first active layer, a p-type spacer layer, a second active layer, and a second n-type semiconductor multi-layer mirror formed on the substrate. First and second current confinement structures are formed in the vicinity of the first and second active layers, respectively, a p-side electrode is electrically connected to the p-type spacer layer, a first n-side electrode is electrically connected to the first n-type semiconductor multi-layer mirror, and a second n-side electrode is electrically connected to the second n-type semiconductor multi-layer mirror.
In the above structure, since no p-type semiconductor multi-layer mirror is needed, the resistance of the laser apparatus can be reduced. Accordingly, the operation voltage and consumption power can be reduced. Further, where a plurality of light-emitting devices are arranged in the laser apparatus, a structure of the common-anode type can be constructed by putting the p-side electrodes of the respective devices on an equipotential level.
Where first and second current confinement portions are formed in the vicinity of the first and second active layers, respectively, resistors of the current confinement portions having a high resistance are connected in a parallel form. Therefore, the resistance of the laser apparatus can be further decreased.
The current confinement structure near the active layer can be established by forming an ion-injected region in a peripheral portion of the device. This structure can preferably be formed as follows. A first p-type current confinement layer including a first Al-containing semiconductor layer is formed between the first active layer and the p-type spacer layer, a second p-type current confinement layer including a second Al-containing semiconductor layer is formed between the second active layer and the p-type spacer layer, and the first and second Al-containing semiconductor layers are selectively oxidized, respectively. The current structure can be flexibly established with good controllability by this method.
Specifically, the substrate can be an n-type semiconductor substrate, and the first n-type semiconductor multi-layer mirror can be electrically connected to the first n-side electrode through the substrate. In such a structure, where a plurality of light-emitting devices are arranged, the first n-side electrodes and the p-side electrodes of the devices can be readily connected electrically, respectively.
The substrate can also be a semi-insulating-type semiconductor substrate. In this case, the first n-type semiconductor multi-layer mirror can include a contact layer whose portion is exposed, the n-side electrode can be formed on the exposed portion, and the first n-type semiconductor multi-layer mirror can be electrically connected to the first n-side electrode through the contact layer. In such a structure, where a plurality of light-emitting devices are arranged, the first n-side electrodes of the devices can be readily connected electrically.
It is preferable to determine layer number, material, composition and thickness of the spacer layer such that the first and second active layers are positioned at loops of electric-field standing waves present in the cavity structure, respectively.
Further, it is preferable to determine layer number, material, composition and thickness of the first and second p-type current confinement layers such that the first and second Al-containing semiconductor layers are positioned at nodes of electric-field standing waves present in the cavity structure, respectively.
The spacer layer can preferably be formed of alternately-layered semiconductor layers of a high doping concentration and semiconductor layers of a low doping concentration. In this case, layer numbers, materials, compositions and thicknesses of the semiconductor layers of high and low doping concentrations are preferably determined such that the semiconductor layers of a high doping concentration are positioned at nodes of electric-field standing waves present in the cavity structure, respectively.
A third n-type current confinement layer including a third Al-containing semiconductor layer can be interposed between the first n-type semiconductor multi-layer mirror and the first active layer, and/or between the second n-type semiconductor multi-layer mirror and the second active layer. In this structure, another current confinement structure consisting of the third Al-containing selectively oxidized semiconductor layer is formed in the third p-type current confinement layer.
The present invention is also directed to a method of fabricating the above surface emitting laser apparatus having at least a light-emitting device, in which the first n-type semiconductor multi-layer mirror, the first active layer, the first p-type current confinement layer including the first Al-containing semiconductor layer, the p-type spacer layer, the second p-type current confinement layer including the second Al-containing semiconductor layer, the second active layer, and the second n-type semiconductor multi-layer mirror are formed on the substrate; the semiconductor layers are etched until a side of the first p-type current confinement layer is exposed to form a pole-shaped light-radiation region; the first and second Al-containing semiconductor layers are oxidized toward a central portion of the light-radiation region to form the current confinement structures above and below the spacer layer; a peripheral portion of the light-radiation region is removed until a surface of the spacer layer is exposed to form a step structure; and the p-side electrode is formed on the exposed surface of the spacer layer.
The present invention is further directed to a method of fabricating the above surface emitting laser apparatus having at least a light-emitting device and using a semi-insulating-type semiconductor substrate, in which the first n-type semiconductor multi-layer mirror including the contact layer, the first active layer, the first p-type current confinement layer including the first Al-containing semiconductor layer, the p-type spacer layer, the second p-type current confinement layer including the second Al-containing semiconductor layer, the second active layer, and the second n-type semiconductor multi-layer mirror are formed on the semi-insulating-type substrate; the semiconductor layer are etched until a surface of the contact layer is exposed to form a pole-shaped light-radiation region; the first and second Al-containing semiconductor layers are oxidized toward a central portion of the light-radiation region to form the current confinement structures above and below the p-type spacer layer; a peripheral portion of the light-radiation region is removed until a surface of the p-type spacer layer is exposed to form a step structure; the p-side electrode is formed on the exposed surface of the p-type spacer layer; and the first n-side electrode is formed on the exposed surface of the contact layer.
The present invention is also directed to a method of driving the above surface emitting laser apparatus having at least a light-emitting device, in which the first and second n-side electrodes are electrically connected to put the first and second n-side electrodes at an equipotential level; and one of current injection and voltage application is performed between the first and second n-side electrodes and the p-side electrode to drive the surface emitting laser apparatus.
The present invention is further directed to a method of driving the above surface emitting laser apparatus having at least a light-emitting device, in which one of current injection and voltage application is performed between the first n-side electrode and the p-side electrode; and one of current injection and voltage application is performed between the second n-side electrode and the p-side electrode independently of the first step to drive the surface emitting laser apparatus.
The present invention is further directed to a method of driving the above surface emitting laser apparatus having at least a light-emitting device, in which one of bias-current injection and bias-voltage application is performed between the first n-side electrode and the p-side electrode; and one of modulated-current injection and modulated-voltage application is performed between the second n-side electrode and the p-side electrode independently of the first step to modulate an oscillation condition of the surface emitting laser apparatus.
The present invention is also directed to a method of driving the above surface emitting laser apparatus of a common-anode type having a plurality light-emitting devices, in which the first n-side electrodes of the light-emitting devices are electrically connected to put the first n-side electrodes on an equipotential level; one of common bias-current injection and common bias-voltage application is performed between the first n-side electrodes and the p-side electrodes for the light-emitting devices; and one of modulated-current injection and modulated-voltage application is performed between the second n-side electrode and the p-side electrode for each light-emitting device to modulate oscillation conditions of the respective surface emitting devices independently.
The present invention is further directed to a method of driving the above surface emitting laser apparatus which has a plurality light-emitting devices, has the first n-type semiconductor multi-layer mirror including the partly-exposed contact layer, the first n-side electrode is formed on the exposed portion, and the first n-type semiconductor multi-layer mirror is electrically connected to the first n-side electrode through the contact layer. In this driving method, the p-side electrodes of the light-emitting devices are electrically connected to put the p-side electrodes on an equipotential level; and a ratio between a current injected between the first n-side electrode and the p-side electrode and a current injected between the second n-side electrode and the p-side electrode, or a ratio between a voltage applied between the first n-side electrode and the p-side electrode and a voltage applied between the second n-side electrode and the p-side electrode is changed for each light-emitting device to drive the respective light-emitting devices independently.
These advantages, as well as others will be more readily understood in connection with the following detailed description of the preferred embodiments and examples of the invention in connection with the drawings.